In recent years, a switching device that withstands relatively high frequency current and voltage have been developed. A switching power supply apparatus having such a switching device that can relatively easily convert a commercial power supply into a DC power has become widespread.
FIG. 1 is a block diagram showing an example of such a switching power supply.
In FIG. 1, reference numeral 11 represents a connector that is connected to a commercial power supply. Reference numeral 12 represents an input filter that removes noise that the switching power supply generates. Reference numeral 13 represents a rectifying circuit that has a rectifying diode that rectifies an AC power supply into a DC voltage (Vin).
Reference numeral 14 represents a transformer (T1) that has a primary winding N1, a secondary winding N2, and a tertiary winding N3. Reference numeral 15 represents a load circuit (unit or apparatus). An AC power is induced in the secondary winding of the transformer (T1) 14. A rectifying diode D3 and a smoothening capacitor C2 convert the AC power into a DC output voltage (Vo). The voltage (Vo) is supplied to the transformer (T1) 14.
The load circuit 15 has a secondary battery cell. When the load circuit is in an operation stop state, the load circuit charges the secondary battery cell. In contrast, when the load circuit is in an operation state, the secondary battery cell supplies a required power to the load circuit. The load circuit is for example a digital camera, a video camera, a small TV, or the like.
Reference numerals 16 and 17 represent operational amplifiers (OP1) and (OP2) that detect an output voltage (Vo) and an output current (Io), respectively. Reference numeral 18 represents a photo coupler (PH1) that has a photo diode and a photo transistor into which detection signals of the operational amplifiers 16 and 17 are input through diodes D1 and D2. Outputs of the operational amplifiers (OP1) and (OP2) become a signal for detecting means that detects a load power. The signal is supplied from the photo diode to the photo transistor of the photo coupler (PH1) and then to an FB terminal of a controlling circuit 19 as a control signal that turns on and off a transistor Q1 as a switching device.
The switching device Q1 can be composed of an MOS FET.
A power induced in the tertiary winding N3 is supplied to the controlling circuit 19 through a rectifying circuit. The controlling circuit 19 is normally composed of an IC circuit. The rectifying circuit is composed of a diode D4 and a smoothening capacitor C1.
Next, the operation of the foregoing switching power supply will be described.
The AC power supply is rectified and a DC voltage (Vin) is obtained. The DC voltage (Vin) is applied to a starting resistor Rp. As a result, a weak current is supplied to the controlling IC circuit through the starting resistor Rp. When a voltage (Vcc) of the controlling circuit 19 rises to an operation region of the controlling circuit 19, it outputs drive pulses at an oscillation frequency of for example 100 kHz. The drive pulses cause the switching device Q1 to turn on and off a current that flows in the primary winding N1 of the transformer (T1) 14.
In this example, it is assumed that the power supply is a fly-back type power supply. When the switching device Q1 is turned on, electromagnetic energy is stored in the primary winding N1. When the switching device Q1 is turned off, the electromagnetic energy stored in the primary winding N1 induce powers in the secondary winding (N2) and the tertiary winding (N3) of the transformer (T1) 14.
The output voltage of the foregoing switching power supply is controlled as follows. The voltage induced in the secondary winding (N2) is rectified by the diode D3 and the smoothening capacitor C2. The output voltage (Vo) is input to a minus terminal of the operational amplifier (OP1) 16. In addition, a reference voltage REF1 is input to a plus terminal of the operational amplifier (OP1) 16. The operational amplifier (OP1) 16 compares the voltage Vo with the reference voltage REF1 and outputs an error signal. The error signal is supplied to the photo coupler (PH1) 18 through the diode D1.
The error signal of the voltage is transferred from a secondary side to a primary side of the photo coupler (PH1) 18. The controlling circuit 19 has a pulse width modulating (PWM) circuit that controls the ON period of the switching device Q1 disposed on the primary side of the transformer (T1) 14 so as to control the power on the secondary side.
As a result, with the reference voltage REF1 of the operational amplifier (OP1) 16 as the reference voltage on the secondary side, the output voltage is controlled.
On the other hand, an output current (Io) that flows to the load circuit 15 flows in a resistor R1 that has a low resistance. The current that flows in the resistor R1 is converted into a voltage. The voltage is applied to a plus terminal of the operational amplifier (OP2) 17 through a reference voltage REF2.
A minus terminal of the operational amplifier (OP2) 17 is connected to the other terminal of the R1 terminal through the reference voltage REF2. The operational amplifier (OP2) 17 compares the current amount of the reference voltage REF2 with the current that flows in the resistor R1.
The operational amplifier (OP2) 17 compares the current amount corresponding to the reference voltage REF2 with the current amount that flows in the resistor R1 and outputs an error signal. The error signal is input to the photo coupler (PH1) 18 through the diode D2. Like the case that the voltage is controlled, the controlling circuit (IC circuit) 19 on the primary side controls the on/off rate of the switching device Q1 so that the output current Io becomes a predetermined current amount corresponding to the reference voltage REF2.
As described above, the operational amplifier (OP1) 16 controls the output voltage Vo for a predetermined voltage and the operational amplifier (OP2) 17 controls the output current Io for a predetermined current. The operational amplifier (OP1) 16 and the operational amplifier (OP2) 17 compose detecting means.
Next, based on the foregoing operations, the non-load operation of which the output current (Io) does not flow in the load circuit 15 will be described.
When the load current (Io) flows, the controlling circuit 19 (PWM IC controlling circuit) control a repetitive oscillation at a predetermined basic oscillation frequency for example 100 kHz. The controlling circuit 19 controls the ON period of the switching device Q1 corresponding to the load power in accordance with the PWM.
On the other hand, in the non-load state, the controlling circuit 19 controls an oscillation at a low frequency in the minimum pulse period as will be described later. FIG. 2 shows waveforms of the base and collector of the switching device Q1 at those timings.
When the load current flows, the waveform of the base of the switching device Q1 oscillates at f1 (for example, 100 kHz). However, in the non-load state of which the pulse width becomes the minimum, the oscillation frequency lowers. The waveform of the base of the switching device Q1 oscillates at f2 (for example, 20 kHz). In other words, in the non-load state, drive powers for the operational amplifiers 16 and 17, the photo coupler 18, and the controlling circuit 19 are required. In other words, the ON period of the switching device Q1 is fixed and the OFF period thereof is varied so that the output voltage is controlled for a predetermined voltage. As a result, the OFF period of the switching device Q1 is varied so as to lower the oscillation frequency.
Japanese Patent Laid-Open Publication No. HEI 10-14217 discloses a technology for a PFM (frequency lowering) control in a low load state (the amount of the load is detected). However, the related art does not describe an ON pulse width, an AC input voltage, and so forth necessary for the PFM control.
Next, with reference to FIG. 3 and FIG. 4, a problem with respect to such a control will be described.
FIG. 3 shows the relation of waveforms of a detection signal (the voltage at the FB terminal) of a load power that is output from the photo coupler 18 and the collector voltage and the base voltage of the switching device Q1 at timing of which a current that flows in the secondary winding on the load side.
FIG. 3 shows the case that the load apparatus that operates in the state that the maximum load current (for example, 2 A) flows on the secondary side changes into the standby mode. When the load current gradually decreases, the voltage at the FB terminal gradually lowers from a high voltage. As a result, the load apparatus operates in the Min load state.
In the period, the power conversion of the power supply operates at the basic frequency (for example, 100 kHz). The switching apparatus performs a pulse width converting operation (PWM control) of which only the ON period of the switching device is shortened.
As the load current decreases, the voltage at the FB terminal lowers. When the FB voltage value becomes equal to or lower than the level of the frequency variation (VCO) start voltage (in this example, 1 V), the apparatus performs a frequency varying operation for lowering the switching frequency. At that point, the DC voltage V3 of the tertiary winding start lowering. According to an embodiment of the present invention, in the non-load state (standby state), the apparatus stably performs the non-load operation at an oscillation frequency of 1.5 kHz.
Next, with reference to waveforms shown in FIG. 4, the case that the load abruptly varies will be described.
When the apparatus that operates in the maximum operation state (for example, 2 A) is turned off, the load current abruptly decreases. When the apparatus performs the non-load operation of which the load current is zero, the voltage drop against the maximum load current is superimposed with the output voltage (at a point of which the load current becomes zero). As a result, the voltage on the secondary side instantaneously becomes equal to or higher than the control output voltage (Vo). Thus, the apparatus temporarily stops the power conversion. At that point, as shown in FIG. 4, the FB (feedback) voltage that is a signal that controls the power conversion abruptly drops from the H level to the L level. In addition, the FB voltage becomes lower than the level of the output stop voltage. Thus, the power conversion stop (switching frequency stop) takes place.
In other words, at that point, since the oscillation stops, the output (an ON pulse of the switching device Q1) of the power conversion IC becomes the OFF state.
Since the power conversion stops, the voltage (V3) of the tertiary winding gradually lowers. The voltage V3 lowers to an IC operation stop voltage (for example, 9 V) of the controlling circuit 19.
Even if the FB voltage becomes higher than the output stop voltage, when the voltage V3 becomes the IC operation stop voltage, the operation of the IC circuit of the controlling circuit stops.
Thus, the voltage of the controlling IC becomes equal to or lower than the operation stop voltage. As a result, the starting circuit starts the starting operation again.
When the start current flows in the controlling circuit 19 through the Rp resistor shown in FIG. 1 and a predetermined time period elapses, the voltage of the IC circuit of the controlling circuit 19 becomes in the operation range, 16 V according to the embodiment. At that point, the output voltage starts rising. The rectifying voltage V3 of the tertiary winding rises to 16 V or higher as shown in FIG. 4. As a result, the IC starts operating. At that point, the IC circuit starts the power conversions of the secondary winding and the tertiary winding.
Since the controlling circuit 19 that uses the IC circuit has the foregoing operation characteristic, after the IC circuit that composes the controlling circuit 19 stops until the IC circuit starts, when the apparatus on the secondary side is controlled and turned on, the load current abruptly increases as denoted by a dotted line. However, since the controlling circuit 19 stops, it does not transfer the power to the secondary winding and the tertiary winding. As a result, when the power is turned on, the output voltage Vo lowers. After the controlling circuit 19 starts, the output voltage Vo gradually rises. Thus, the apparatus cannot start operating in the period.
When the load apparatus starts operating, if the output voltage of the power supply lowers, a reset error of a system microcomputer of the apparatus on the loading side or the apparatus may unstably operate.
In recent years, the current consumption of apparatus has been decreased. An apparatus that does not almost consume a load current in the standby state has been commercialized. When the output voltage drops, the standby mode changes to the operation stop mode. As a result, a device that stores the operation state of the apparatus will be reset.
When a recording operation is periodically performed by a video camera or the like, after the recording operation is stopped, if the video camera becomes a zero current mode such as a standby mode so as to reduce power consumption, the operation is reset. As a result, the recoding operation cannot be resumed.
An object of the present invention is to provide a switching power supply apparatus that allows an output voltage to be stably supplied against abrupt load variation such as a startup and stop of a load apparatus.